1. Field of the Invention
This invention relates to coupling assemblies which are controllable to vary the amount of torque transmitted by the assembly. Such assemblies find particular, but not exclusive, use in the drive transmissions of motor vehicles.
2. Description of the Prior Art
A viscous shear coupling, as described for example in United Kingdom Patent Specification No. 1,357,106 (and corresponding U.S. Pat. Ser. No. 3,760,922 issued 25 Sept. 1973) comprises a housing part, a hub part within the housing part, the parts being relatively rotatable about a common axis, and two sets of interleaved plates. The plates of one the sets are rotatable with the housing and the plates of the other set are rotatable with the hub. The plates of one of the sets are axially movable into contact with the plates of the other set. The housing contains a viscous liquid such, for example, as a silicone oil.
Normally torque is transmitted between the housing part and the hub part due to shearing forces in the viscous liquid between adjacent plates, i.e. in viscous mode. The above patent specifications describe that the housing part may be filled with viscous liquid to a fill factor which leaves part of the housing normally filled with air or gas.
Such couplings exhibit a phenomenon which has come to be known as the "hump" made in which the plates of the two sets come into contact and torque is transmitted by metallic friction and/or mixed friction between the plates.
Prolonged slipping of the coupling in "viscous mode" causes heating and expansion of the silicone oil. When the oil has expanded to fill the entire space within the coupling there is a sharp increase in pressure in the coupling and, due to the throttling effect between adjacent plates, the plates of one set are moved into contact with the plates of the other set by the fluid pressure in the coupling.
The onset of the hump mode is therefore dependent upon a temperature rise in the coupling producing a pressure increase therein. The decay of pressure in the coupling is slow with decrease in temperature and therefore separation of the plates after being in the hump mode will take some time.
There have been various proposals to vary the torque-transmitting characteristics of viscous couplings by varying the fill factor and/or by varying the spacing between the plates of the two sets when the coupling is in use. These proposals have only succeeded in varying the torque transmitting characteristics of the coupling within narrow limits and do not enable the torque transmitting capability of the coupling to be significantly increased or decreased at will.
A particular application where it is desirable to be able to control the torque transmission at will is in a so-called viscous transmission of a motor vehicle. In such a transmission, as shown for example in FIG. 17 of the above mentioned patents, one axle of the vehicle is driven directly from the prime mover while the other axle is driven through a viscous coupling only when there is a significant speed difference between the axles. This can provide problems where the front axle of the vehicle is directly driven, the rear axle is driven through the viscous coupling and the front wheels are heavily braked so that there is a considerable speed difference between the front and rear axles. The braking torque will be transmitted through the viscous coupling to the rear wheels and will tend to brake these. If the rear wheels lock lateral stability of the vehicle is impaired. If the vehicle is fitted with an anti-lock braking system (hereinafter called an ABS system) then transfer of braking torque from the front wheels to the rear wheels will prevent proper operation of the system. It would be desirable, therefore, in such configurations to be able substantially to disable the viscous coupling but with present technology this is not possible and therefore one-way clutches have been proposed for this purpose, see for example U.S. Pat. No. 4,605,087 issued Aug. 12, 1986.